What does "ReferenceError: $ is not defined" mean, and why does in my case it shown

[SiddhiVinayaka]

Javascript:

$(document).ready(function(){
var lentable="";
var table1=document.getElementById("#tablebody");
for(var i=0;i<table1.length;i++)
{
lentable=table1.length[i];
console.log("lentable");
}
});

HTML:

 <table>
                <tbody id="tablebody">
                    <tr>
                        <td>Completed</td>
                        <td>Completed</td>
                    </tr>
                    <tr>
                        <td>Completed</td>
                        <td>Completed</td>
                    </tr>
                    <tr>
                        <td>Completed</td>
                        <td>Completed</td>
                    </tr>
                </tbody>
            </table>

[Dormilich]

did you import the jQuery library?